1. Field of the Invention
The present invention relates to an optical fuse, an optical fuse complex, and an optical fuse system using the optical fuse, which prevents damage to a light receiver such as a photo detector by immediately intercepting a beam in response to an unexpectedly high intensity entering an optical system.
This application is based on Japanese Patent Applications Nos. 10-070751 and 10-084027, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Rare earth element doped optical fiber amplifiers, which can extend transmission distances by directly amplifying a signal beam, have been indispensable for a long distance mass communication system. These rare earth element doped optical fiber amplifiers excite the rare earth element doped in an optical fiber with pumping light at wavelengths shorter than the wavelengths of the signal beam to be transmitted, and amplify the signal beam by induced emission.
These rare earth element doped optical fiber amplifiers may damage or deteriorate a subsequent optical device on a light path because of the high power output, thus lowering the stability and reliability of the optical communication system.
In particular, when going from a no signal condition to an input of the signal beam, a secondary pulse which contains a high intensity light component (hereinafter referred to as "light surge") may damage the optical device.
The light surge occurs for the following reason. When no external beam is input, the rare earth element doped optical fiber amplifier has been excited by excitation light at wavelengths shorter than those of the signal beam. Because the rare earth element has been excited, light energy is stored in the rare earth element doped optical fiber amplifier. When the signal beam is input to the rare earth element doped optical fiber amplifier, the energy stored in the rare earth element doped optical fiber amplifier is suddenly output by the induced emission, thus amplifying the signal beam by a light gain.
A technique for suppressing the light surge has been proposed. Japanese Patent Application, First Publication No. 6-216452 notes the cause of the light surge and discloses a technique for making the gain constant even when a signal beam is input to the rare earth element doped optical fiber amplifier in which energy has been stored.
The light surge occurs because of the sudden induced emission of the energy stored in the rare earth element doped optical fiber amplifier. When no signal beam is input, the technique inputs a dummy beam at wavelengths different from those of the signal beam, making the intensities of the signal beam and the dummy beam equal and preventing the light surge.
FIG. 22 is a block diagram showing the structure of the background art of Japanese Patent Application, First Publication No. 6-216452. An optical multiplexer 201 multiplexes the dummy beam (wavelength: .lambda.3) with the signal beam (wavelength: .lambda.1). A optical branching device 205 branches a part of the transmitted power, and a receiver 204 performs optical/electric conversion (O/E conversion). A dummy beam semiconductor laser emission controller 203 performs feedback to a dummy beam semiconductor laser 202 so as to set the voltage after the O/E conversion to a fixed value.
The light multiplexer 206 multiplexes the multiplexed beam and excitation beam (wavelength: .lambda.2) emitted by the excitation semiconductor laser. The second multiplexed beam enters the rare earth element doped optical fiber amplifier 210 through an optical isolator 209, and is amplified during the transmission through the rare earth element doped optical fiber amplifier 210. The beam from the rare earth element doped optical fiber amplifier 210 travels through a second optical isolator 211. An optical filter 212 allows only the signal beam .lambda.1 to travel.
Although this technique prevents generation of light surges without interception of the signal beam, the technique cannot protect an optical device once a light surge accidentally occurs. In particular, the number of the rare earth element doped optical fiber amplifiers has increased recently. In systems using this technique, all the amplifiers require structures for emitting and controlling the dummy beam, thus increasing the costs.
Japanese Patent Application, First Publication No. 9-146056 discloses an optical fuse and a method for suppressing light surges, in which a semiconductor device with an optical waveguide (for example, a compound semiconductor) or a rare earth element doped optical fiber is inserted in a post stage of the rare earth element doped optical fiber. The compound semiconductor optical waveguide device deteriorates by the light surge. That is, because the semiconductor absorbs light, the end of the waveguide generates heat, and burns out instantaneously. With this characteristic, the device works as an optical fuse for a light surge. The device is advantageous because the absorption wavelength band or threshold value of the absorption may be adjusted by controlling the combination of materials (solid solution composition, or a super lattice cycle). However, there is the problem that the number of the semiconductor waveguide device which may be damaged increases the costs of systems.
Japanese Patent Application, First Publication No. 9-146056 discloses a process for connecting a rare earth element doped optical fiber using light absorption caused by transitions of inner electrons of a rare earth element such as Er. The process produces a device which undergoes reversible destruction. Other similar reports which describe absorption of light of a high intensity using rare earth elements, in particular, Er are available. In recent examples, "Y. Maeda, Appl. Phys. Lett., Vol. 72 (4), 1-3" (1998) discloses that, in a wavelength band of 790 nm, low intensity light is not absorbed but high intensity light is effectively absorbed by lutetium aluminum garnet doped with Er. This phenomenon is strongly related to transition probability and relaxation probability of electron levels of Er ion. Depending on the number of electrons at a specified level (for example, in an I.sub.13/2 condition of Er), the absorption wavelength band may be controlled. The control may be achieved by adjustment of atom positions of the rare earth element in a matrix material (amorphous SiO.sub.2 in an optical fiber, or garnet), excitation of the electrons by temperature control, or control of the transition probability and relaxation probability. To perform these processes, high material design technology is needed, increasing the costs of the products. Further, the optical system may cause a problem at low temperatures at which electrons are not present in a preselected wavelength band.
Japanese Patent Application, First Publication No. 9-244074 discloses a light limiter circuit which includes an optical non-linear element whose transparency loss is changed depending on the intensity of an incident beam. The reference does not provide detailed description of the elements which perform the light limiting operation of the main point of the circuit. The optical limiting element using a non-linear optical material may utilize a process applying Cherenkov radiation, which is a non-linear effect, or a process applying parametric amplification which is another non-linear effect. These processes provide a non-destructing-type (reversible-type) device suppressing a light surge. However, the non-linear optical constant is small, depending on the preselected wavelength band, and it is necessary to extend the length of the element (the length of the waveguide) to compensate for insufficient effects. Therefore, the size and costs of the device cannot be reduced when a number of devices suppressing light surges are employed.
As described above, there are three kinds of methods for suppressing light surges: (a) the method for directly preventing generation of light surges; (b) a method using the reversible physics in response to only a light surge (high intensity beam); and (c) the method using the device whose material is damaged by the light surge. A disposable optical fuse which reduces the costs according to the method (c) has recently been in demand.